To provide interactivity with the interface, you use buttons.

To add a button, click the "Add button" button in the toolbar, and then drag a rectangle with the mouse onto the GUI. You will see it displayed as a text button, with the text "New button" on. Notice that the Properties window is now displaying properties for your new button rather than the GUI.

Using the Properties window, you can set a picture for the button instead, and you can also set various other self-explanitory attributes. You set what happens when the player clicks on the button by using the "Click Action" attribute. This can be set to "Run Script" (the default), and also "Set mode", which changes the cursor mode to the mode specified in the "New mode number" property.

To delete a GUI button, right-click it and choose Delete.